Alloy,article of manufacture,and process

ABSTRACT

ARTICLES OF MANUFACTURE GENERALLY AND IN PARTICULAR TAPPETS FOR INTERNAL COMBUSTION ENGINES ARE MADE OF A HIGH CARBON HIGH CHROMIUM ALLOY CONTAINING FROM ABOUT 1.3% TO ABOUT 3.1% CARBON, FROM ABOUT 15% TO ABOUT 35% CHROMIUM WITH THE REMAINDER IRON, WITH OR WITHOUT UP TO ABOUT 3.25% SILICON, MANGANESE AND OTHER RESIDUALS. THE ALLOY IS CAST, COOLED SO QUICKLY THAT A RELATIVELY SMALL NUMBER OF RELATIVELY LARGE PRIMARY CHROMIUM CARBIDE PARTICELS ARE FORMED AND WIDELY DISPERSED IN A MATRIX OF AUSTENITE CONTAINING A SOLID SOLUTION OF CHROMIUM AND CARBON. THEN LARGE NUMBERS OF RELATIVELY SMALL PARTICLES OF CHROMIUM CARBIDES ARE PRECIPITATED FROM THE MATRIX AND DISTRIBUTED THROUGOUT THE SPACES BETWEEN THE LARGE PRIMARY CARBON PARTICLES LEAVING THE REMAINDER OF THE MATRIX CONTAINING CARBON AND SUSCEPTIBEL TO SUBSEQUENT HARDENING. THEN THE CASTING IS HARDENED BY HEATING AND SUBSEQUENT QUENCHING AT SUCH TEMPERATURE AND AT SUCH TIME THAT THE MATRIX IS SUBSTANTIALLY CONVERTED TO MARTENISTE WITHOUT SIGNIFICANTLY CHANGING THE CARBIDE PARTICLES.

Sept. 12, 1972 E. A. THOMPSON ALLOY, ARTICLE OF MANUFACTURE, AND PROCESS Original Filed Feb. 24. 1966 2 Sheets-Sheet l FIG 1 FIG 2 EAR L A THOMPSON s s E c 0 R P D m N: m T C MA m T. A w m fl s .YO mm m d e n F m g a O 2 shetm z FIG '5 FIG 4' FIG 7 H mm FIG 6 INVENTOR, EARL A THOMPSON Attorney United States Patent Ofiice' 3,690,959 Patented Sept. 12, 1972 3,690,959 ALLOY, ARTICLE OF MANUFACTURE, AND PROCESS Earl A. Thompson, Bloomfield Hills, Mich., assignor to F. 105. Lamb Company, Warren, Mich.

Original application Feb. 24, 1966, Ser. No. 520,690, now Patent No. 3,502,057, dated Mar. 24, 1970. Divided and this application Mar. 18, 1970, Ser. No. 20,517

Int. Cl. C22c 37/06; F01] 1/14 US. Cl. 1482 Claims ABSTRACT OF THE DISCLOSURE Articles of manufacture generally and in particular tappets for internal combustion engines are made of a high carbon high chromium alloy containing from about 1.3% to about 3.1% carbon, from about to about 35% chromium with the remainder iron, with or without up to about 3.25% silicon, manganese and other residuals. The alloy is cast, cooled so quickly that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon. Then large numbers of relatively small particles of chromium carbides are precipitated from the matrix and distributed throughout the spaces between the large primary carbon particles leaving the remainder of the matrix containing carbon and susceptible to subsequent hardening. Then the casting is hardened by heating and subsequent quenching at such temperature and at such time that the matrix is substantially converted to martensite Without significantly changing the carbide particles.

This application is a division of my application Ser. No. 529,690 filed Feb. 24, 1966, now Pat. 3,502,057.

This invention relates to high-chromium, high-carbon iron alloys, to processes for making such alloys, and to articles made of them. Such alloys are useful for a wide variety of articles in which hardness and high resistance both to corrosion and wear are important. The invention is particularly, but not exclusively suitable for hydraulic tappets or lash adjusters for internal combustion engines, which are disclosed herein, in no sense by way of limitation, but as illustrating one way in which the invention may be practiced.

An example of the general construction of a tappet showing the relationship of the parts which make my alloys especially suitable for them, is in my US. Pat. 2,805,352, Dec. 23, 1958. Such a tappet customarily includes an assembly which is inserted in compression in the valve train, and is constantly urged to expand. This takes up lost motion when there is no load on the valve train, that is when the valve operating mechanism lets the valve close. When the valve is being opened, the tappet is urged to collapse by the force of the cam in opening the valve against the force of the valve closing spring, but collapse is retarded so that the tappet becomes substantially rigid at the operating speed, and the cam can be effective to open the valve.

Such devices often have a cup-shaped plunger sliding in a cup-shaped tappet body, a spring constantly urging the plunger out of the body to expand the tappet, and an oil trap between the plunger and the body to retard collapse. The rate of collapse is determined by the clearance between the plunger and the body, through which clearance, oil is expressed from the trap. It is important that this clearance be maintained within critical limits over long periods of time, because even a small amount of wear can seriously enlarge the oil discharge passage, and this increases the rate of escape of oil from the trap which in turn increases the rate of collapse of the tappet and thus delays the valve opening and reduces the amount of openmg.

Consequently it is very important to provide a tappet in which neither the body nor the plunger can wear significantly over long periods of use, and which therefore maintains the desired clearance or fit.

In addition to wear due to friction the clearance is often seriously affected by corrosion of the plunger or of the body of both. This corrosion can be caused by contact with various corrosive substances which form in the engine lubricating oil. The condition is particularly aggravated by running the engine cold, which happens when cars are used for short runs which prevents the engine from becoming warm enough to operate properly.

Many attempts have been made to solve this problem, but none has been completely successful. For example two proposals have been to form the tappet parts of supposedly corrosive resistant stainless steel and to plate iron or steel tappet parts with chromium. The known stainless steels are so hard to work that it is prohibitively expensive to make tappet parts from them and some are not as resistant to corrosion as they should be in an engine. While plating may retard corrosion. I have found that the corrosive reagents found in many cars under many operating conditions soon destroy the plating, leaving the tappet, particularly vulnerable to wear and accelerated corrosion.

This invention is based in part on my discovery that greatly improved articles can be cast of particular highchromium, high-carbon iron alloys which can be treated to provide a surprisingly easily machinable article, which article can be processed further to give it surprising hardness and resistance both to wear and to corrosion. The article is surprisingly stable as to dimension in the hardening step, so that it can be machined with customarily cutting tools and ground to very nearly finished size and shape. This requires a minimum of finish grinding after subsequent hardening. Such alloys, so treated, are suitable for a wide variety of uses. They make surprisingly longlasting, corrosion-free tappets.

The dimensional stability throughout changing temperature in heat treatment indicates that the tappet is also stable over long periods of use, an advantage which has become evident during prolonged use in engines. This prevents the change of dimension or growth, sometimes observed in customary engine tappets, a growth which changes the clearance between the tappet body and plunger and thus affects the operation of the engine.

Accordingly one of the objects of the invention is to provide an improved article of manufacture which is easily machinable and is highly resistant to wear and corrosion, and is dimensionally stable, both during manufacture and 1n use.

More specifically, another object is to provide a tappet in which the body or plunger or both is made of an im proved material which is extremely resistant both to corrosion and to wear.

Another object is to provide a tappet which can be economically made by conventional processes and which has improved dimensional stability.

Another object is to provide an improved process for making improved articles of general use, which contributes to economical manufacture of precision articles of high corrosion and wear resistance.

Other objects and advantages of the invention will be understood fromthe following description and from the annexed drawings, in which I FIG. 1 is a longitudinal section of a valve tappet to which my invention is applied.

FIG. 2 is one-half of a symmetrical, longitudinal section of a mold for casting tappet bodies in accordance with my invention.

FIG. 3 is a similar half section of a symmetrical mold for casting plungers for tappets in accordance with my invention.

FIG. 4 is a photograph of a polished and etched section corresponding to FIG. 1, of a portion of a tappet made of an alloy embodying one form of my invention. This photograph is of metal in the condition as cast, and is magnified about 1333 times. The scale line, approximately A; of an inch long at the bottom of the photograph represents one ten thousandth of an inch (.0001).

FIG. 5 is a photograph corresponding to FIG. 4 of the same alloy after a subsequent heat treatment.

FIG. 6 is a photograph corresponding to FIG. 4 of the same metal after subsequent hardening.

FIG. 7 is a photograph corresponding to FIG. 4 of the same metal after drawing following hardening.

Referring to FIG. 1, 10 designates generally a tappet of known form which may be urged upwardly as FIG. 1 is seen by a conventional cam 12 to push upward a push rod 14 for opening a valve as is known. The tappet or automatic lash adjuster may include a cup-shaped body having a tubular portion 20 closed at its lower end by a foot or base 22 and slidable in a suitable bore 24 in the engine 26. Slidable in the tubular portion is the cupshaped plunger 28 which supports at its upper end a push rod seat 30 which receives the push rod 14. The push rod seat 30 is piloted in the bore of the body 20 to eliminate any possible side thrust on the plunger 28 from the push rod 14.

The plunger is constantly urged upward or out of the cup by compression spring 32 and it is positively held in the cup by a snap ring 34.

Oil is supplied to the inside of the tappet from a gallery 36 in the engine and through passage 38, and through passages 39 and 40 in the walls of the cups. An oil trap is formed in the space 42 below the plunger. When the valve closes, the spring 32 pushes the plunger up to take up slack and this reduces pressure in the oil trap below pressure in the gallery 36. Consequently oil enters the oil trap through the check valve 44 as is known. When the cam starts to open the valve, it tends to collapse the tappet and creates pressure in the oil trap 42 greater than the pressure in the gallery 36. This tends to force oil out of the oil trap in the clearance between the outer surface of the plunger 28 and the inner surface of the body 20. The rate at which oil can escape through this clearance is what determines the rate of collapse of the tappet and it affects the timing and amount of opening of the valve. The clearance is provided between an accurately formed and highly polished cylindrical outer surface of the plunger and a similar precise cylindrical inner surface on the body. It is this clearance which must be maintained precisely over long periods of time.

The preferred material from which I make either the body 20 or the plunger 28 or both is a high carbon, high chromium iron alloy containing about 2.20% carbon and about 22.5% chromium. This alloy may also contain about 1.60% silicon and about .90% manganese. The silicon may be added to make the alloy easier to pour. The manganese combines with any sulphur which may be present in the material of which the alloy is made. Also the manganese may improve the hardenability of the matrix of the alloy upon quenching. Ordinarily such alloys are made from available ingredients including scrap or pig iron of uncertain analysis so that the resulting alloy may contain residual quantities of copper, nickel, molybdenum and other metals. As one example an analysis of one batch of my preferred alloy showed 2.20% carbon, 1.60% silicon, .90% manganese, 22.5% chromium, and residuals of .25% copper, .31% nickel and .17% molybdenum.

The silicon, manganese and the residuals amount to about 3.25%, and I believe that these do not importantly affce the final metallurgical structure, for the purposes of my invention. Consequently alloys containing them come within my definition of an alloy having stated ranges of 4 carbon and chromium, and having the remainder principally iron.

The tappet may be cast in a mold as shown in FIG. 2. This is one half of a symmetrical section of a mold which has a central casting passage 50, lower runner passages 52 for pouring the metal of the feet 22 (when this is a different metal from that in the body, as will be explained), upper passages 54 for pouring the metal of the tubular portions 20, and a plurality of cavities 56 in which the tappet bodies are formed between mold sections 58 and core sections 60. The core has a central opening 62 to help the casting cool quickly.

The plunger may be cast in a mold as shown in FIG. 3 which includes an integral mold and core section 70 having a core opening 72 and a top 74 which completes the mold. The metal 76 for the plunger is cast into a pour passage 78 corresponding to the passage 50 and fills a number of mold cavities each of which provides a plunger casting 80.

I have discovered that alloys of the composition mentioned above, or of the ranges of composition disclosed herein, can be given a new and improved metallurgical structure by cooling quickly after pouring, and that this new metallurgical structure can be treated to provide new, surprising and very desirable properties. As one example a melt having the proportions of ingredients to provide the alloy of the composition set forth above was poured at about 2750 F. This particular alloy has a liquidus temperature of about 2399 F. and a solidus temperature of about 2270 F., as determined by the Leeds and Northrup carbon determinator.

FIG. 4 is a photograph of a portion of the bottom wall of a plunger as shown in FIG. 1 which has been cast according to my invention. The temperature of this casting has been reduced from the liquidus to the solidus so quickly that two things have happened. One is that the usual formation of primary chromium carbide particles has been arrested, so that the chromium carbide particles formed are fewer in number and smaller than they would be if the metal had cooled slowly. Evidence of this is that the matrix has remained essentially non-magnetic austenite. If the casting had cooled slowly, austenite would not be formed. The other thing that has happened is that the matrix contains large amounts of chromium and carbon in solid solution. 'Evidence of this is the subsequent formation of very fine chromium carbide particles during subsequent heat'treatment. If the casting had cooled slowly the carbon and chromium now remaining in solution would have precipitated out as primary carbides. The primary chromium carbides shown in FIG. 4 are very small, much smaller than if the casting had cooled slowly, also they are more widely dispersed. The largest primary carbide particle visible in FIG. 4, measured in inches is about .00135 long, and in a representative area .001 square there are about 17 primary carbide particles. The large dark particles shown are what is generally called chromium carbides. Among such chromium carbides Cr C and Cr C have been identified. It is also possible for iron was cooled under the following conditions.

FIG. 2 shows a plunger and mold about two times actual size. Six plungers each weighing approximately two-thirds of an ounce, having'an outside diameter of approximately of an inch, a length of about 1%; inch, a side wall thickness of about .060 and a bottom wall thickness of about .090 were cast of the alloy mentioned above in a 6-part shell mold, each part having the proportions shown and formed of silicon sand bound with 3% phenolic resin binder. The metal was poured at about 2750 F. into molds at room temperature. The thickness of the metal cast and the cooling characteristics of the mold were such that when the casting had cooled below the solidus temperature for this particular alloy, that is about 2270 F. the metallurgical structure shown in FIG. 4, and described above, was formed.

I have found that faster cooling forms even smaller and fewer primary chromium carbide-particles. The thickness of the metal influences the rate of cooling and this influences the metallurgical structure and properties of the cast metal, not only as cast, but in subsequent treatment. For example the thin wall of the plunger cools faster than the thick bottom. There is an important and discernible difference in the appearance and properties of the metallurgical structures of the bottom and side wall, as cast. The side wall can also be drilled with a high speed tool steel drill more easily than the bottom, after the subsequent heat treating step described below. Also after final hardening, as disclosed below, a thicker casting (slowly cooled) is softer than a thinner casting (quickly cooled). For example a tappet body about .190 thick as cast, cooled as described above, will have an ultimate hardness of about 60 Rockwell C, whereas a body having .160 thickness and cooled as described will have a final hardness of about 68 Rockwell C. The open space inside the core accelerates cooling.

I may affect the cooling in other ways. Since a thick section cools more slowly than a thin section it may be necessary to mold thicker sections in zircon sand, for example, which cools the casting faster than silicon sand. Alternatively chills may be placed in the mold to accelerate the cooling of certain thick parts of a casting, or I may use a permanent mold, water cooled. If the metal cools too slowly the casting will not only be too hard, but it cannot be satisfactorily heat treated so as to be machinable.

The important thing is that the temperature of the metal must be reduced from the liquidus to the solidus so quickly that only relatively small numbers of very small chromium carbides can form, and that they will be formed in an austenite matrix which has large inter-carbide spaces in which larger numbers of still smaller chromium carbides can be precipitated upon re-heating, while leaving the matrix containing carbon and in a condition which can be hardened. FIG. 4 shows a typical structure, which has properly cooled according to my invention.

After cooling the plunger casting was heat treated as follows. Its temperature was slowly raised from room temperature to about 1600" F. The time required was three hours. It was held at 1600 F. one hour. It was cooled to about 1400 F. during the next 40 minutes. It was cooled to about 1300 F. during the next hour. Total time /3 hours.

FIG. 5 shows a plunger casting after this treatment, It shows that the chromium carbides of FIG. 4 have not changed significantly. The interstices or inter-carbide spaces in the previously austenitic matrix are now substantially filled with a dispersion of very small precipitated chromium carbides, having a representative size of the order of about .000018 (18 millionths of an inch). In a representative area .0001 square there are about 13 of these very small particles, or about 1300 particles in the .001 square containing 17 primary carbide particles. Thus although the primary chromium carbides in FIG. 4 are very small (a large one being of the order of a thousandth of an inch long) they are of the order of from 50 to 100 times as large as the smaller carbides formed in the reheating process. The hardness after re-heating was from 27 to 33 Rockwell C.

I do not know the exact nature of the matrix after reheating shown in FIG. 5. It is magnetic. It contains carbon, so that it can be hardened by subsequent heat treatment which appears to convert the matrix essentially to martensite having properties typical of tool steel.

In the foregoing heat treatment the time required is a function of temperature, a lower temperature requiring a longer time. Also the time and temperature of this reheating step influences the amount of carbon left in the matrix and so affects the subsequent hardenability of the alloy, when hardened as disclosed below.

This particular combination of carbide particles and the characters of the matrix in the two conditions appear to make possible the machinability at one stage of my invention and the hardness at a subsequent stage, combined with the surprising dimensional stability and other properties I have observed.

After the foregoing re-heating treatment the parts can be machined easily and economically with high speed steel tools and surprisingly the parts can be ground almost to the exact final shape and desired dimensions, in the case of tappets. For example the groove for the snap ring 34 is machined in the body 20, the passage 39 is bored and the inner and outer diameters are ground to very nearly the exact finished size. The plunger is bored to form the passage 40, the passage for the check valve 44 may be bored, if it has not been cast, the valve seat may be coined, and the outer surface is ground to very nearly its exact final size.

Thereafter the part may be hardened by holding at a temperature above the critical temperature at which the matrix changes back into austenite and well below the melting point, followed by quenching. The time is a function of temperature, lower temperature requiring longer time. For example the part may be held at about 1750' F. For about twenty minutes, then oil quenched. FIG. 6 shows a plunger which has been cooled, then reheated, then hardened as above described. The Rockwell C hardness is about 63 to 65. The two sets of chromium carbide particles have remained unchanged. The matrix has been essentially converted to martensite. I find that this hardening step does change the size of the part so slightly that a plunger, for example, holds its diameter to a change of less than .0001 of an inch. As contrasted with this process, the heat treatment of iron customarily used for tappets causes the casting to grow.

Tappets require extremely close tolerances in the difference between the inside diameter of the body and the outside diameter of the plunger. Consequently in assembling a tappet, the plunger to be combined with a particular body must be carefully selected from among several categories of ranges of size. This requires tappet parts to be classified in ranges of size of a few millionths of an inch. Consequently in manufacturing tappets it' is desirable to do a finish grinding operation after hardening. However my invention makes possible grinding before hardening so close to final size that the minimum amount of material need be removed in the final grinding step. In the case of articles which are acceptable within tolerances as large as .0001 (one hundred millionths) of an inch, I can grind to final size before hardening. This is of great advantage in manufacturing.

After hardening, the part may be drawn by holding it at a temperature higher than it will ever work in service, for example of about 375 F., for about one hour. The hardness drops about 1 point Rockwell C and the structure is as shown in FIG. 7, with the alloy discussed above.

The advantages of the invention are realized while varying the proportions of the ingredients of the alloy within the limits stated herein. For example, I may use carbon up to 2.38% and chromium up to 27.00% without significantly changing the characteristics of the alloy from those of the preferred analysis given above, for my purposes.

Increasing the proportion of carbon within certain critical limits tends to increase the final hardness and hence wear resistance of the article. More carbon is required in articles having a thick section, because due to slower cooling, more carbon is combined with chromium, which has a very high affinity for carbon. If more carbon were not used, the matrix would be so depleted that it could not be hardened satisfactorily. More carbon than about 2.95% appears to render the article impractically difficult to machine although in some instances I can use up to about 3.10% carbon, particularly with high percentages of chromium. Increasing the proportion of chromium within a wider range of critical limits tends to increase the corrosion resistance and reduction of the chromium content below about appears to reduce the corrosion resistance undesirably. Increasing the propor tion of chromium beyond about 30% appears to have no important effect on either wear or corrosion resistance, except with very high carbon percentage (above 3.10% for example) and increase of chromium beyond about 35% appears to have no advantage, and may even be undesirable. There is desirable relationship between the amounts of carbon and chromium to have the desired effects because one part carbon will combine with about ten parts chromium. Therefore higher proportions of chromium require higher percentages of carbon so as to leave in the matrix, after the re-heating step, enough carbon not combined with chromium, to harden the matrix satisfactorily in the hardening step discussed above.

For example with my preferred alloy first mentioned, the processes described appear to leave about 1.10% of carbon in the matrix after the first re-heating step (in which the smaller carbide particles are formed). Then when the part is hardened as described, the matrix appears to contain no free carbon and is hardened to have properties resembling those of tool steel or 52,100 steel. Measurements of properties of the cast and hardened alloy exceed those of steel. For example, a sample of the preferred alloy, cast and treated as above described showed a transverse bending stress of 693,000 pounds per square inch. From this the modulus of elasticity is calculated at 39,000,000. The modulus for steel is about 29,000,000.

Many of the advantages of the invention are present in a range of carbon between 1.70% and 2.85% while using a range at chromium between 15 and 27%.

In articles having different parts requiring different hardness, I find it of advantage to use an alloy having the general characteristics described above but being even harder and hence even more wear resistant. In such case I may use a carbon content of about 3.10% and may use this with a chromium content varying between about 30% and about 35%. This provides an extremely hard, wear resistant material. It is ditficult to machine by cutting tools, and although it is difficult to grind I have found that by confining this material to the foot of the tappet body I can satisfactorily machine the inside of the tubular part of the body and grind the exterior surface. This is partly due to my improved casting process which permits casting of two different metals within very small tolerances, and confines the extremely hard alloy to a small amount, making it possible for me to make a tappet to finished size with a minimum of grinding. It is also due in part to the unusual dimensional stability of the material which makes it possible to grind to close tolerances before the hardening step of the manufacturing process described above, and to finish grind by removal of the minimum amount of material. An example of such article is a composite tappet illustrated herein made by the process disclosed in my patent in Great Britain No. 991,513 published May 12, 1965, the disclosure of which is incorporated herein by reference with the same effect as if quoted completely herein. In such case the hard alloy containing about 3.10% carbon and about 27 to 30% chromium is cast in the mold first, to form the foot or bottom 22 of the tappet which contacts the customary cam. The remainder, or tubular member of the tappet body may then be cast of any of the other alloys disclosed herein. The two alloys are autogenously joined along a bonding or juncture zone, or joint, 46.

I have successfully cast various articles having unusually high resistance to corrosion and wear and having exceptional dimensional stability of the alloys having the following analyses.

O 81 Cu Mn Cr N1 M0 I claim as my invention:

1. A corrosion-resistant, hard, dimensionally stable article of manufacture formed of an iron alloy containing from about 1.3% to about 3.10% carbon and from about 15% to about 35% chromium with the remainder iron, the alloy having a minimum hardness ofabout 61 Rockwell C and having a relatively small number of relatively large primary chromium carbide particles distributed in a matrix of martensite and having a relatively large number of relatively small precipicated chromium carbide particles distributed throughout the matrix between the large primary carbide particles.

2. An article of manufacture as defined in claim 1 further characterized by a carbon content between about 1.7% and about 2.85 and a chromium content between about 15% and about 27%.

3. An article of manufacture as defined in claim 1 further characterized by a carbon content between about 2.2% and about 2.35 and a chromium content between about 22% and about 27%.

4. An article of manufacture as defined in claim 1 further characterized by a carbon content of about 2.2% and a chromium content of about 22.5%.

5. A hydraulic valve tappet for an internal combustion engine comprising in combination a plunger member slidable in a cup-shaped tappet body which has a tubular body member, at least one of said members being cast of an iron alloy containing between about 1.3% and about 3.1% carbon and between about 15% and about 35% chromium with the remainder iron, the alloy having a relatively small number of relatively large primary chromium carbide particles distributed in a matrix of martensite and having a relatively large number of relatively small precipitated chromium carbide particles distributed throughout the matrix between the large primary carbide particles.

6. A tappet as defined in claim 5 further characterized by a carbon content between about 1.7% and about 2.85% and a chromium content between about 15 and about 27%.

7. A tappet as defined in claim 5 further characterized by a carbon content between about 2.2% and about 2.35% and a chromium content between about 22% and about 27%.

8. A tappet as defined in claim 5 further characterized by a carbon content of about 2.2% and a chromium content of about 22.5%.

9. The method of making an article which includes pouring a molten iron alloy containing from about 1.30% to about 3.10% carbon and from about 15% to about 35% chromium with the rest iron, rapidly reducing the temperature from the liquidus to the solidus at such a rate that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon then precipitating large numbers of relatively small particles of chromium carbides from the matrix and distributed throughout the spaces between the large primary carbide particles leaving the remainder of the matrix containing carbon and susceptible to subsequent hardening then hardening the casting by heating and subsequent quenching, the last mentioned heating being as such temperature for such time that the matrix is substantially converted to martensite when quenched without significantly changing the carbide particles.

10. The method of making a corrosion resistant, hard dimensionally stable part of a hydraulic tappet for an internal combustion engine which includes pouring a molter. iron alloy containing from about 1.30% to about 3.10% carbon and from about 15% to about 35% chromium with the rest iron, rapidly reducing the temperature from the liquidus to the solidus at such a rate that a relatively small number of relatively large primary chromium carbide particles are formed and widely dispersed in a matrix of austenite containing a solid solution of chromium and carbon, then precipitating large numbers of relatively small particles of chromium carbide from the matrix and distributed throughout the spaces between the large primary carbide particles leaving the remainder of the matrix containing carbon and susceptible to subsequent hardening, then forming the part to a predetermined size and shape by removing material from its surface, then hardening the casting, while converting the matrix to martensite without significantly changing the carbide particles.

References Cited UNITED STATES PATENTS 2,015,991 10/1935 Breeler 123-188 AAX 2,051,415 8/1936 Payson 123-188 AAX 3,078,194 2/1963 Thompson 148-3 X 3,028,479 4/ 1962 Tauschek 123-188 AAX 2,073,178 3/1937 Rich 123-188 AAX 2,127,245 8/1938 Breeler 123-188 AAX 1,245,552 11/1917 Becket 75-126 1,956,014 4/1934 Fink et al. 123-188 2,773,761 12/1956 Fugua et al 148-35 X 2,199,096 4/ 1940 Berglund 148-35 X OTHER REFERENCES Alloys of Iron and Chromium, vol. II, Kinzel et al., 1940, McGraw-Hill C0., New York, pp. 182, 183, 230- 235, 244, 249 and 258.

Chromium in Cast Iron, Electro Metallurgical C0.,

20 1939, pp. 29-37 and 42.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

